I have an iPhone 4 and know people with the 4S. Any IS that the phone could have or that could be emulated via software could not possibly match the IS you're getting from sensor-shift IS or in-lens IS. Definitely not 2-5 stops better.

In my experience with the iPhone 4, low light is the Achilles Heel of the camera and even a 100% improvement would not give you much more low-light freedom, especially if anything is moving.
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